The resister greatly reduces the "ringing" of the high voltage path from the coil to the plug, this ringing is what causes the R.F. generation, the resistor does effect the ignitions characteristics, mostly reducing the duration of the spark.

The old booster gap was fairly effective in reducing plug fouling in some engines that were prone to it, the availability of modern oils that produce less deposits eliminated the need for the booster gap for the most part.

The introduction of Capacitive Discharge ignitions systems with their very fast secondary voltage rise time did more than anything in reducing plug fouling in 2 stroke engines, Capacitive Discharge systems tend to produce more R.F. interference that older type of ignitions which is why more shielding is required when using them in close proximity of radio equipment.